1. Field of the Invention
The present invention relates to a semiconductor device with a thin film transistor (hereinafter referred to as TFT) formed of a semiconductor film that has a crystal structure and is formed on a substrate, and to a method of manufacturing the semiconductor device.
2. Description of the Related Arts
A display for displaying text and images is indispensable means for people to digest information with various semiconductor devices that have semiconductor elements, such as television sets, personal computers, and cellular phones. CRTs have long been in the market to acquire the position of representative display. On the other hand, liquid crystal displays and other flat displays (flat panel displays) are lately increasing their shares exponentially, for electronic devices are desired to reduce weight and size.
One mode of flat panel displays is active matrix driving in which a TFT is provided in each pixel or dot and data signals are sequentially written to display an image. A TFT is an indispensable element in active matrix driving.
Most TFTs are formed from amorphous silicon. Those TFTs cannot operate at high speed and therefore they are used only as switching elements provided in respective dots. Since the TFTs cannot make other elements than switching elements, external ICs (driver ICs) mounted by TAB (tape automated bonding) or COG (chip on glass) are used in data line side driving circuits for outputting video signals to data lines and in scanning line side driving circuits for outputting scanning signals to scanning lines.
However, mounting a driver IC is considered as a limited method because the pixel pitch is reduced as the pixel density is increased. For instance, at a pixel density of UXGA level (1200xc3x971600 pixels) in the RGB color method, at least 6000 connection terminals are necessary even by a crude estimation. An increase in number of connection terminals leads to increased occurrence of contact failure. It also leads to an increase in area of the border surrounding the pixel portion (called a picture frame region), which hinders reduction in size of a semiconductor device that employs this display and spoils the external design of the semiconductor device. Against this background, apparently a display device in which driving circuits are integrated with a pixel portion is needed. By integrally forming a pixel portion and a scanning line side and data line side driving circuits on the same substrate, the number of connection terminals can be markedly reduced as well as the area of the picture frame region.
The integrated driving circuits are demanded to have high driving performance (ON current: Ion) and to improve their reliability by preventing degradation due to the hot carrier effect whereas low OFF current (Ioff) is required for the pixel portion. A lightly doped drain (LDD) structure is known as a TFT structure capable of reducing the OFF current value. In the LDD structure, an LDD region, which is lightly doped with an impurity element, is placed between a channel formation region and a source region or drain region heavily doped with an impurity element. A structure that is known to be effective in preventing degradation of ON current value due to hot carriers is an LDD structure in which an LDD region partially overlaps a gate electrode (gate-drain overlapped LDD; hereinafter abbreviated as GOLD).
A TFT is manufactured by layering a semiconductor film and an insulating film or conductive film while using photo masks to etch the films into given shapes. If optimization of TFT structures to suit what are demanded for the pixel portion and the driving circuits is dealt with by simply increasing the number of photo masks, the manufacture process becomes complicated and the number of steps is increased inevitably.
The present invention has been made to solve those problems, and an object of the present invention is therefore to provide a technique of obtaining TFT structures optimal for driving conditions of a pixel portion and driving circuits using a small number of photo masks.
In order to attain the above object of the invention, the present invention adopts a gate electrode of two-layer structure in which a first layer in contact with a gate insulating film is longer in channel length direction than a second layer. The two-layer structure gate electrode is used in forming source and drain regions and an LDD region in a self-aligning manner in an n-channel TFT of a driving circuit portion. Source and drain regions and an LDD region of an n-channel TFT in a pixel portion are formed not in a self-aligning manner but by using a photo mask. The LDD region of the n-channel TFT in the driving circuit portion is positioned so as to overlap the gate electrode whereas the LDD region of the n-channel TFT in the pixel portion is placed outside of the gate electrode (so as not to overlap the gate electrode). The source and drain regions and the two types of LDD regions, which have different positional relation with respect to the gate electrodes, are formed through two doping treatment steps.
As described above, a method of manufacturing a semiconductor device in accordance with the present invention is characterized by comprising the steps of:
forming on a first insulating film first through third semiconductor films that are separated from one another;
forming a first electrode, a second electrode, and a third electrode respectively on the first semiconductor film, the second semiconductor film, and the third semiconductor film with a second insulating film interposed between the electrodes and the films, the electrodes having a first shape;
using as masks the first shape first through third electrodes in first doping treatment to form first concentration impurity regions of one conductivity type in the first through third semiconductor films;
forming second shape first through third electrodes from the first shape first through third electrodes;
forming through second doping treatment a second concentration impurity region of the one conductivity type in the second semiconductor film, and third concentration impurity regions of the one conductivity type in the first semiconductor film and the second semiconductor film, the second concentration impurity region overlapping the second shape second electrode; and
forming through third doping treatment a fourth concentration impurity region and a fifth concentration impurity region in the third semiconductor film, the regions having the other conductivity type that is opposite to the one conductivity type. In other words, the semiconductor device manufacturing method of the present invention is characterized in that etching treatment for forming a gate electrode of a TFT is combined with doping treatment to form LDD regions and a source or drain region in a self-aligning manner.
Further, according to another structure of the present invention, the method is characterized by comprising the steps of: forming on a first insulating film a first semiconductor film, a second semiconductor film, and a third semiconductor film that are separated from one another; forming a first shape first electrode above the first semiconductor film with a second insulating film interposed therebetween; using the first shape first electrode as a mask to form a first concentration impurity region of one conductivity type in the first semiconductor film; forming a first shape second electrode and third electrode above the second semiconductor film and the third semiconductor film with the second insulating film interposed between the semiconductor films and electrodes; etching the first shape second electrode and third electrode to form a second shape second electrode and third electrode; forming through second doping treatment a second concentration impurity region of the one conductivity type in the second semiconductor film, and third concentration impurity regions of the one conductivity type in the first semiconductor film and the second semiconductor film, the second concentration impurity region overlapping the second shape second electrode; and forming through third doping treatment a fourth concentration impurity region and a fifth concentration impurity region in the third semiconductor film, the regions having the other conductivity type that is opposite to the one conductivity type.
With the manufacture method as this, LDD that overlaps a gate electrode is formed in a self-aligning manner in an n-channel TFT of a driving circuit. This LDD is obtained at the same time a source or drain region is formed through the same doping step by utilizing the film thickness difference (level difference) of the gate electrode. On the other hand, a mask is used to form LDD that does not overlap a gate electrode in an n-channel TFT of a pixel portion.
The term semiconductor device in the present invention refers to devices in general that utilize semiconductor characteristics to function, and display devices, representatively, liquid crystal display devices having TFTs, and semiconductor integrated circuits (micro processors, signal processing circuits, high frequency circuits, and the like) are included therein.